1. Field of the Invention
The present invention relates to a differential transmission circuit and an information processing system using the differential transmission circuit, and more particularly, to a technique of suppressing unintentional electromagnetic radiation from the differential transmission circuit.
2. Description of the Related Art
A transmission line is used in high-speed digital transmission of serial data. For example, not single-end transmission but differential transmission is generally used to transmit serial data at a speed of hundreds or more of Mbit/s. A transmission line used for the differential transmission is a differential transmission line. In general, the differential transmission line includes a grounded conductive layer and a pair (two) of transmission line conductors which is formed on one side of the grounded conductive layer via a dielectric layer interposed therebetween and extends in a strip shape.
For example, a driving circuit performing driving at a predetermined bit rate outputs a differential transmission signal to the differential transmission line. Since the ideal differential transmission signal is different from a clock signal and does not include a repeated signal pattern, the ideal differential transmission signal has no large peak intensity on the frequency spectrum. However, in effect, switching noise occurs due to non-linearity of a transistor in the driving circuit. As a result, a large peak is sometimes observed as noise in the frequency corresponding to the bit rate of the serial data at which the driving circuit performs the driving and harmonics of the frequencies, that is, frequencies which are natural number multiples of the frequency corresponding to the bit rate of the serial data at which the driving circuit performs the driving in a frequency spectrum of a common mode signal component of the differential transmission signal output to the differential transmission line. The frequency corresponding to the bit rate of the serial data refers to the frequency of 10 GHz when the serial data is binary modulation serial data of, for example, the bit rate of 10 Gbit/s. A part of the switching noise in these frequencies of the common mode signal component propagates as radiation loss toward to the surroundings during the transmission of the differential transmission signal through the differential transmission line, and thus the unintentional electromagnetic radiation occurs in these frequencies.
In techniques of the related art, propagation of electromagnetic waves to a space is suppressed by covering the differential transmission line and the driving circuit by a metal case and allowing a gap (or a hole) formed in the assembly of the metal case to be very small and the propagation of the electromagnetic waves to a space is further attenuated by disposing an electromagnetic wave absorption material inside the metal case. Further, Patent Document 1 (JP2006-332302A) and Non-Patent Document 1 (“Chip Common Mode Choke Coils Supporting USB 3.0 SuperSpeed Signals”, Murata Manufacturing Company, Ltd., “http://www.murata.co.jp/new/news_release/2010/0625/index.html” (searched for on Mar. 31, 2011) disclose techniques of suppressing the propagation of a common mode signal component by inserting a common mode choke coil into the differential transmission line to increase the impedance for the common mode. Further, Patent Document 2 (JP2010-41228A) discloses a technique of suppressing generation of switching noise by innovation of a differential output circuit of a differential amplification IC, suppressing the conduction propagation of the common mode signal component, and thus suppressing the occurrence of the unintentional electromagnetic radiation.